The present invention pertains generally to methods and systems for the hydrothermal treatment of a feed stream to destruct waste, recovery heat, or produce beneficial chemicals. More specifically, the present invention pertains to methods and systems for the hydrothermal treatment of organics which contain inorganic compounds such as salts or oxides or which will generate these inorganic compounds. The present invention is particularly, but not exclusively, useful as a method and system for the hydrothermal treatment of organics under supercritical temperature and pressure conditions, or at supercritical temperatures and elevated, yet subcritical pressures.
It is well known that a broad spectrum of materials can be chemically treated in an aqueous media at either supercritical temperature and pressure conditions, or at supercritical temperatures and elevated, yet subcritical pressures. In supercritical water oxidation (xe2x80x9cSCWOxe2x80x9d), the oxidation reaction occurs substantially entirely at conditions which are supercritical in both temperature ( greater than 374xc2x0 C.) and pressure ( greater than about 3,200 psi or 220 bar). Specifically, at temperatures of about five hundred degrees Celsius to six hundred fifty degrees Celsius (500xc2x0 C.-650xc2x0 C.) and pressures of about 250 bar, rapid and complete oxidation of virtually any organic compound can be obtained in an aqueous media in a matter of seconds. A process related to SCWO known as supercritical temperature water oxidation (xe2x80x9cSTWOxe2x80x9d) can provide similar oxidation effectiveness for certain feedstocks but at pressures as low as 25 bar. In both of these processes, the temperature and pressure can be varied to accommodate the type of feedstream and the desired result. For example, these processes can be used to combust materials of high thermal value for energy recovery; to convert hazardous waste materials into more benign materials; or to produce beneficial chemicals for later use. In general, these processes involve combining water, a reactant, and an oxidizer such as air or oxygen, at elevated temperatures and pressures. The resultant chemical reaction is generally exothermic and occurs directly within the aqueous phase. The energy released by the reaction can often be used to maintain the high temperatures and pressures required in the reactor vessel. By continuously feeding the reactants while withdrawing the reaction products, the energy released from the reaction can be used to heat the incoming feedstream. Batch type processing is generally inefficient in these processes due to the large amount of energy that would be needed to heat and pressurize each batch.
The various processes for oxidation in an aqueous media at temperatures above about three hundred seventy-four degrees Celsius and pressures above about 25 bar are referred to collectively as hydrothermal treatment. In addition to the increased reaction rates as described above, other reaction features distinguish hydrothermal treatment from reactions conducted at standard temperatures and pressures (STP), which are generally considered to be 25 degrees Celsius and 1.013 bar. For example, most inorganic salts have high solubility""s in water at STP. In stark contrast, under hydrothermal treatment conditions, most inorganic salts are insoluble in the aqueous media. Consequently, inorganic salts that are present in the feedstreams precipitate from the aqueous media and create solids. These solids can be problematic because they often buildup on the surfaces of process equipment such as the walls of the pressure vessel used to contain the reaction. In continuous feed processes, the buildup of solids often progresses until the reactor vessel becomes plugged. Once the reactor vessel is plugged, the continuous reaction must be interrupted to clean out the reactor vessel, wasting valuable time and energy.
Further complicating hydrothermal treatment is the fact that corrosion rates generally increase with increasing temperature. Feedstreams used for hydrothermal treatment often generate corrosive acids such as hydrochloric acid and sulfuric acid, resulting in corrosive attack on the process vessel that is so severe that alkali is often added to neutralize the acids. Unfortunately, this addition of alkali creates insoluble salts which aggravate the vessel plugging problem described above. Further, stress considerations often dictate that the reactor vessel have a relatively narrow diameter and long length to thereby withstand the high pressures and corrosion rates generating in the reactor, yet reactor vessel""s with narrow diameters further aggravate the plugging problem.
The extreme temperatures, pressures, corrosives and insoluble salts present in the hydrothermal reactor vessel present what can only be characterized as a harsh environment to the pressure bearing wall of the reactor vessel. To alleviate the effects of this environment on the pressure bearing wall, liners have been heretofore suggested to separate the reactor chamber from the pressure bearing wall. For example, U.S. Pat. No. 5,591,415 which issued to Dassel et al. entitled xe2x80x9cReactor for Supercritical Water Oxidation of Wastexe2x80x9d discloses a reactor enclosed in a pressure vessel in a manner that the walls of the pressure vessel are thermally insulated and chemically isolated from the harsh environment of the reaction zone. Unfortunately, the liner disclosed by Dassel et al. fails to adequately address the problem associated with insoluble salt buildup and reactor plugging. Similarly, U.S. Pat. No. 3,472,632 which issued on Oct. 14, 1969 to Hervert et al. entitled xe2x80x9cInternally Lined Reactor for High Temperatures and Pressures and Leakage Monitoring Means Thereforexe2x80x9d discloses a liner having a porous layer for a high temperature reactor. Hervert et al., however, does not disclose the use of the liner for hydrothermal treatment environments, and consequently, the disclosed liner lacks several very important features necessary for using a liner in hydrothermal treatment. For instance, the liner disclosed by Hervert et al. is not a suitable mechanism for relieving the effects of insoluble salt buildup and reactor plugging, it is not easily replaceable, and there is no thermal barrier.
In light of the above, it is an object of the present invention to provide a liner to protect the pressure bearing wall of a hydrothermal treatment reactor wherein the liner includes a system for leak detection that is operable during the hydrothermal reaction which allows for reactor shutdown before a severe attack on the pressure bearing wall occurs. Another object of the present invention is to provide a liner to protect the pressure bearing wall of a hydrothermal treatment reactor wherein the liner incorporates a mechanism for pre-heating the reaction chamber before steady state treatment conditions are achieved. Yet another object of the present invention is to provide a liner to protect the pressure bearing wall of a hydrothermal treatment reactor wherein the liner incorporates a mechanism for passing a heat exchange fluid near the reactor chamber to allow heat to be recovered from the reaction. Still another object of the present invention is to provide a liner to protect the pressure bearing wall of a hydrothermal treatment reactor incorporating a mechanism to control the liner temperature and thereby prevent the buildup of insoluble salts on the liner. Yet another object of the present invention is to provide a system and method for accomplishing hydrothermal treatment which is easy to implement, simple to use, and cost effective.
In accordance with the present invention, a system for performing hydrothermal treatment at temperatures above approximately three hundred seventy-four degrees Celsius (374xc2x0 C.) and pressures above about 25 bars, includes a reactor vessel that is formed with a pressure bearing wall which surrounds a reactor chamber. An inlet is provided at one end of the reactor vessel to introduce the feed material into the reactor chamber and an outlet is provided at the other end of the reactor vessel to allow the reaction products to be withdrawn from the reactor chamber.
The surface of the pressure bearing wall that faces the reactor chamber is covered by a liner to protect the wall from exposure to temperature extremes, corrosives and salt deposits. For a cylindrically shaped reactor vessel, the liner is cylindrically shaped having a first end and a second end, and conforms to the inside surface of the reactor vessel. The liner is formed with three layers: a non-porous, corrosion resistant primary layer; a porous layer; and a non-porous, secondary layer. The porous layer is positioned between the primary layer and the secondary layer. The liner is positioned in the reactor vessel with the secondary layer facing the pressure bearing wall of the reactor vessel and the primary layer facing the reactor chamber. Seals are provided at each end of the liner. Each seal extends from the primary layer to the secondary layer to thereby seal the porous layer between the primary layer and the secondary layer. The secondary layer of the liner can be placed directly against the pressure bearing wall of the reactor vessel, or a gap can be left between the liner and the pressure bearing wall of the reactor vessel. When a gap is used, a hole may be provided in the wall of the reactor vessel to allow fluid flow in the gap and, therefore, pressurization of the gap. Additionally, provision can be made for fluid communication between the gap and the reactor chamber. An optional layer of insulation can be selectively interposed between the secondary layer of the liner and the pressure bearing wall of the reactor vessel to insulate the wall of the reactor vessel.
A connector extending through the pressure bearing wall or the closures (ends) of the reactor vessel and through the secondary layer of the liner is provided to allow fluid communication between the porous layer and a pump located outside the reactor vessel. When activated, the pump allows a heat transfer fluid to be pumped into the porous layer for circulation within the porous layer. A similar second connector passing through the wall and secondary layer provides an exit for the heat transfer fluid circulating within the porous layer. The discharged heat transfer fluid that flows out of the second connector can be piped back to the pump for recirculation or to a storage reservoir.
In addition to the connectors used for pumping the heat transfer fluid, one of the heat transfer fluid connectors, or another connector may be provided in the wall of the reactor vessel to allow for sampling of the fluid within the porous layer. Specifically, the purpose of this sampling is to determine whether a leak has developed in the corrosive layer of the liner. To do this, the physical or chemical properties of a sample may be measured by a sensor. Physical and chemical properties that may be useful for this purpose include: fluid pressure; fluid flow; fluid temperature; and detection of the presence of a particular chemical species in the fluid. For the present invention, the leak detection connector can function in at least two different ways. In one configuration, a sensor can be positioned within the porous layer allowing the connector to function as a conduit to relay a signal from the sensor to a recorder/display. Alternatively, the connector can function as a fluid passageway allowing the fluid from the porous layer to flow through the connector to an externally located sensor. In either case, the connectors allow for leak detection measurements to be performed during the hydrothermal treatment of the reactants thereby ensuring the continuous integrity of the corrosion resistant layer of the liner.
For the present invention, partitions can be positioned within the porous layer, with each partition extending from the corrosion resistant layer to the pressure bearing wall. Thus, the partitions divide the porous layer into sections and isolate the sections from each other. If partitions are used, separate connectors can be provided for each section to thereby allow each section to be independently heated, cooled and monitored for leaks.
In one embodiment of the present invention, the porous layer is used during installation of the liner in the reactor vessel. Specifically, a cold fluid is passed through the porous layer of the liner to cool and therefore contract the liner prior to insertion of the liner in the reactor chamber. In this manner, a liner can be constructed having an initial outside diameter that is slightly greater than the inside diameter of the reactor vessel. When constructed and installed in this manner, a liner having a tight fit with the reactor vessel can be obtained. To remove the liner from the reactor vessel, a cold fluid can again be passed through the porous layer of the liner to re-contract the liner.
In operation, a warming fluid can be selectively passed through the porous layer of the liner to pre-heat the reactor chamber during periods preceding steady state treatment conditions. Additionally, a coolant can be selectively passed through the porous layer of the liner during the hydrothermal treatment of the reactants to cool the corrosion resistant layer of the liner and to maintain the pressure bearing wall at low service temperatures. By maintaining the temperature of the corrosion resistant layer of the liner at sub-critical temperatures, corrosion rates can be reduced and the accumulation of inorganic solids on the liner can be prevented. Also in accordance with the present invention, the connectors can be utilized in performing leak detection measurements during the hydrothermal treatment of the reactants to ensure the continuous integrity of the corrosion resistant layer of the liner.